1. Field of the Invention
The present invention relates to a bar code reader for reading lower quality bar code symbols, and particularly to a bar code reader for reading bar code symbols that may vary in contrast, from high to medium to low contrast bar code symbols.
2. Description of the Prior Art
In one common technique employed for reading bar code symbols, a continuous analog signal representing the reflectance function of a bar code symbol is digitized by presenting the continuous analog signal to a comparator along with a threshold voltage. The comparator has a first logical level output when the amplitude of the continuous analog signal exceeds that of the threshold, and a second logical level output when the amplitude of the reference threshold exceeds that of the waveform. Several references, for example, U.S. Pat. No. 4,323,772 to Serge, U.S. Pat. No. 4,245,152 to Flurry et al., U.S. Pat. No. 4,219,152 to Couch et al., and U.S. Pat. No. 4,147,295 to Nojiri et al. all describe bar code digiti zing systems of this general configuration.
When the scanned bar code symbol is a high-quality, high contrast bar code symbol, local maxima and minima of the processed analog signal rise to points well above or well below the value of the threshold, and thus are readily digitized by the comparator.
However, when the bar code symbol is a low contrast bar code symbol, local maxima of the analog signal often fail to exceed the value of the threshold. Consequently, comparators employed to digitize continuous analog waveforms representing the output of an image sensor often fail to digitize localized maximum points from low contrast analog signals.
Low contrast of a bar code symbol may result from defective printing or imprinting of a bar code symbol, poor (including excessive or insufficient) lighting conditions, dirt or debris accumulating on a bar code symbol, smudging or running of a bar code, or fading of a bar code symbol. Bar code symbols imprinted on garments that are subjected to repeated washing are especially susceptible to fading and low contrast conditions.
Thus, there is a need for a bar code reader which may or may not include decoding circuitry that may vary in contrast sensitivity in order to digitize bar code symbols as described previously.